As is known in the art, one issue in continuous wave (CW) radars, such as FMCW and interrupted CW, is that part of the transmitted signal leaks through to the receiver and can degrade dynamic range performance. Usually in a CW radar, transmission and reception occur on the same antenna resulting in a direct path feedthrough. However, feedthrough can also occur in radars in which the transmitting antenna is separated from the receiving antenna, the target of interest is far from both, and some of the transmitted signal reflects from a cloud or other object entering the receiver ahead (in time) of the reflected target signal.
Prior art techniques for reducing transmitter feedthrough into the receiver are disclosed in U.S. Pat. Nos. 3,021,521, 4,970,519 and 5,646,625. These techniques rely on some form of direct coupling of the transmitted signal to the receiver where that signal is combined into the receiver in a destructive manner such that the transmitter component that came into the receive antenna is nulled out. A block diagram of a prior art generic feedthrough nulling circuit is shown in FIG. 1. The circuit of FIG. 1 may be acceptable for use in systems with a limited number of receiver channels, where the receiver is physically close to the transmitter, and the received frequency is the same as the transmitted frequency. When the receiver is hundreds of feet away (or more) from the transmitter and when there are many receiver channels used, the prior art system of FIG. 1 becomes impractical due to the number of transmit signals that must be split or routed to all the receiver channels. Such a design would incur a significant cost in amplifiers and/or very long cables if the receive antenna is large and the receivers are dispersed. Also, if the transmitter and the receiver are not on the same frequency simultaneously, the system of FIG. 1 does not work. A similar problem occurs in telecommunications using full duplex radios that must allow simultaneous transmit and receive on different frequencies.
One known attempt to address these types of feedthrough problems lies in separating the different transmit and receive from each other by use multiplexing filters (a.k.a. frequency division multiplexing). An example of its use is shown in FIG. 2 contained in U.S. Pat. No. 4,197,540. In radar receivers, use can also be made of the target Doppler shift which makes the receive frequency different from the transmitted frequency is shown in FIG. 3.